City Research Online

Abstract

The selection of an optimal working fluid and the design of the system components for a small-scale (<100 kW) high-temperature (250 – 400 °C) organic Rankine cycle (ORC) can be challenging owing to the possibility of subatmospheric condensation pressures and high expander volume-ratios. The latter means that volumetric expanders are not suitable, whilst a single-stage turbine would be characterised by supersonic flow and small blade heights. Alternatively, a cascaded cycle can be considered, in which the heat-rejection from a topping cycle drives a bottoming cycle. Through the proper selection of working fluids for the two cycles, sub-atmospheric condensation pressures can be avoided, whilst the volume-ratio is divided across two separate expansions. Moreover, two-phase expansion in the topping cycle can be considered to increase the power output from the system. At present, there are limited guidelines for the selection of fluids for each cycle. In this paper, the development of a novel method to
identify the optimal pairing of fluids for cascaded ORC systems is discussed. The model is comprised of a cascaded ORC thermodynamic model and the Peng-Robinson equation of state. Using this equation of state allows the fluid parameters to be included within the optimisation, which allows the identification of optimal fluid parameters for both the topping and bottoming cycles, alongside the optimal operating conditions. The model has been used to identify optimal fluids for cascaded systems for heat-source temperatures ranging between 250 and 400 °C. The results have been verified by separate optimisation studies completed using REFPROP. Finally, a comparative study has shown that optimal cascaded systems can achieve similar power outputs to simple ORC systems and have lower
expander volumetric ratios. However, cascaded systems require larger heat exchangers. The performance of cascaded systems could be further improved through two-phase expansion, and this should be studied in the future.

Publication Type:	Conference or Workshop Item (Paper)
Publisher Keywords:	Cascaded ORC; Organic Rankine cycle; Waste-heat recovery; Working-fluid selection
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	School of Science & Technology > Engineering

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